1. Field of the Invention
This invention relates generally to hubs for bicycles. More specifically, the present invention relates to improvements in a bicycle unit hub.
2. Description of the Prior Art
As is well known, the rear wheel of a bicycle incorporates a hub which typically comprises a hub shell rotatably supported on a hub shaft by a pair of bearings. The hub shell has a gear mounting portion around one bearing (right-hand bearing) for mounting a freewheel. The freewheel includes an inner ring (core) fixed on the gear mounting portion of the hub shell, and an outer ring (sprocket carrier) arranged around the core via a one-way clutch mechanism to rotate only in one direction relative to the core. The sprocket carrier supports a plurality of diametrically different sprockets in the case of a multiple freewheel.
In the typical rear hub described above, the gear mounting portion must be larger in diameter than the right-hand bearing located in the gear mounting portion, and the sprocket carrier is diametrically larger than the core which, in turn, is diametrically larger than the gear mounting portion. Thus, the applicable minimum diameter for the smallest sprocket supported on the sprocket carrier is correspondingly limited, consequently imposing limitations on the selectable speeds provided by the multiple freewheel.
A unit hub, which is a preassembled unit of a hub and a freewheel, is designed to greatly reduce the applicable minimum diameter for a smallest sprocket. Such a unit hub is increasingly used in various bicycles.
A typical unit hub comprises a hub shell having its left-hand end rotatably supported on a hub shaft via a first bearing, a core mounted to the right-hand end of the hub shell and rotatably supported on the hub shaft via a second bearing, and a sprocket carrier arranged around the core via a one-way clutch mechanism to rotate relative to the core only in one direction. Because the second bearing supports the core instead of directly supporting the hub shell, the gear mounting end (right-hand end) of the hub shell may be greatly reduced in diameter. Thus, the core and sprocket carrier supported on the gear mounting portion can be correspondingly reduced in diameter, thereby enabling a great reduction in the applicable minimum diameter for a smallest sprocket.
In nature, the unit hub is handled as a unit and not intended to be disassembled. Indeed, it may be possible to conduct maintenance, to a certain degree, of the unit hub without disassembly. Thus, in general, the core of the unit hub is irremovably mounted to the hub shell.
In actual use, however, it becomes sometimes necessary to disassemble the unit hub for performing more careful maintenance. In view of such a requirement, Japanese Utility Model Publication No. 1-13605 (Published: Apr. 21, 1989; Inventor: Masashi NAGANO) discloses an improved unit hub which has a general configuration resembling that of the previously described typical unit hub but which enables disassembly when necessary.
Specifically, the improved unit hub disclosed in the above Japanese publication comprises a hub shell rotatably supported on a hub shaft, a core (inner ring) mounted in spline engagement with the right-hand end of the hub shell for co-rotation therewith, and a sprocket carrier (outer ring) arranged around the core via a one-way clutch mechanism to rotate relative to the core only in one direction. The improved unit hub further includes a hollow or cylindrical bolt which is inserted into the hub shell from the right-hand end thereof. The bolt has a threaded tip portion engageable with an internally threaded bore of the hub shell. The bolt further has a head which comes into abutting engagement with the core at a position remote from the hub shell.
When the bolt is tightened up, the core is clamped and fixedly held between the bolt head and the hub shell. The spline engagement between the core and the hub shell enables torque transmission from the core to the hub shell. When the bolt is loosened out of engagement with the threaded bore of the hub shell, the spline engagement allows the core to be removed axially from the hub shell, thereby enabling disassembly for maintenance.
While the improved unit hub enables disassembly of the core from the hub shell, it still has the following problems.
In general, the hub shell is made of a light metal (e.g. aluminum) which is relatively weak. Thus, the threaded bore of the hub shell together with the threaded tip portion of the hollow bolt must have a sufficient axial length to insure an enough clamping force as required for fixedly holding the core even under a large torque. As a result, the bolt must be turned many times for assembly and disassembly of the core relative to the hub shell, which makes it cumbersome for the user to perform maintenance of the unit hub.
Further, the internally threaded portion (bore) of the hub shell must have a sufficient wall thickness to meet strength requirements. Thus, certain limitations are necessarily imposed on the freedom in designing the configuration of the hub shell.